Genetic studies on retrovirus-induced disease and the transmission of infectious virus have led to the identification of numerous chromosomal genes involved in these processes. Genetic mapping of these genes to specific chromosomal loci has been accomplished by the use of somatic cell hybrids alone or in conjunction with classical Mendelian crosses. These studies have now led to the chromosomal mapping of common provirus integration sites found in tumors induced by Gross Passage A MuLV, (Gin-1) Moloney MuLV (Dsi-1) and mouse mammary tumor virus (Int-3). Because these studies have also mapped an increasing number of germline viruses, a revised nomenclature system was derised for one group of endogenous retroviruses, the MMTVs. Use of hybrid cells also permitted the identification of the Bxv-1 provirus and implicated this provirus in the formation of the leukemogenic MCF MuLV. Finally, cell hybrids have been increasingly used to chromosomally map other genes for which cloned probes are available including an interferon receptor, interleukin-3 and the oncogenes Draf, Ets, Rel, and Mbcl-2.